This invention relates to a built-up marine propeller, i.e. a marine propeller the blades of which are detachably secured to a hub body. More particularly, the invention relates to a built-up marine propeller having a hollow hub body and a plurality of propeller blades distributed about the hub body and detachably secured to it in engagement with an external bearing surface on a wall of the hub body.
Although not so limited, the invention is particularly useful in a built-up propeller having adjustable blades, i.e. a propeller the blades of which can be moved to a selected pitch position on the hub body and locked in that position.
In such a propeller the blade pitch is fixed in the sense that it cannot be changed when the propeller is rotating. However, when the propeller is stationary, it is possible to change the pitch within a certain relatively narrow range.
A need for such a small change of the pitch of a marine propeller may occur from time to time as a consequence of changed operating conditions of the ship equipped with the propeller, e.g. in connection with a change from summer operation to winter operation or operation at a changed maximum or cruising speed.
Patent specifications GB-1 455 504 and DE-483 317 and a company publication KaMeWa ABP (Adjustable Built-Up Propeller) published in 1974 by Karlstads Mekaniska Werkstad (Sweden), show some examples of prior art embodiments of built-up marine propellers with detachable blades, each blade being secured to the hub body by a plurality of bolts. In these prior art propellers, the propeller blades are locked to the hub body by means of a number of fastening devices. Each fastening device includes a tension rod in the form of a threaded bolt extending through holes formed in the hub body and in the blade flanges by which the blades are seated on a bearing surface of the hub body.
In the propellers shown in GB-1 455 504, the bolts are stud bolts passed from inside the hub body into threaded blind holes in the blade flanges and tightened by nuts screwed onto the inner ends of the stud bolts. Dowels are used to locate the blades in a precise pitch position relative to the hub body. There is no provision for adjusting the pitch.
In the propeller shown in DE-483 317, the bolts are headed bolts passed from the outer side, the side of the blade flanges exposed to the water, through elongate holes in the blade flanges into the hub body. Although the purpose of the elongate shape of the holes in the blade flanges is not to make the pitch adjustable, it may nevertheless admit of some adjustment of the pitch.
In the propeller shown in the afore-mentioned company publication, headed bolts are passed from inside the hub body into threaded blind holes in the blade flanges. The holes in the blade flanges and the holes in the hub body are arranged such that a few different predetermined pitch positions can be selected. Dowels are used to locate the blades accurately in the different pitch positions. When a blade is to be adjusted, the bolts are first loosened so that the blade can be turned about its longitudinal axis to a desired position within the range of adjustability. The bolts are then tightened to lock the blade in the selected position.
As will be appreciated, it is very important that the blades are firmly locked in the selected position of adjustment. The force by which the bolts have to clamp the blade flange against the bearing surface, and hence the torque with which the bolts have to be tightened, is therefore substantial. Near the bolts the space available for the application of wrenches or other tightening tools is limited, however, and for this and other reasons it is difficult to apply a torque that is great enough to ensure a satisfactory locking relying only on the friction between the blade flange and the hub body. This is so especially where the bolts are tightened from inside the hub body.
The same problem exists in built-up propellers having non-adjustable blades. Even in such propellers, the fastening devices have to be tightened under a considerable torque.
In the prior art propellers the dowels used to accurately locate the blades relative to the hub body and to prevent undesired changes of the pitch after the adjustment also occupy a certain space, and for that reason only a few fixed pitch positions within the range of adjustment are feasible.
It is desirable to be able to adjust the blades substantially continuously between the limits of the range of adjustment, i.e. to an infinite number of positions of adjustment which can be selected as desired, and to lock the blades reliably in every selected position of adjustment without having to use dowels. Whether or not the propeller is of the type having adjustable blades, it is also desirable to be able to apply the fastening devices from within the hub body with adequate force using tools small enough to permit them to be used inside the hub body.
Accordingly, it is an object of the invention to provide a propeller of the kind indicated initially having these desirable properties.
In accordance with the invention this object is achieved with a propeller having the features set forth in the characterising part of the independent claims. The dependent claims are directed to preferred features of the propeller according to the invention.
As will appear more clearly from the description that follows, each tension rodxe2x80x94which may be the shank of a boltxe2x80x94may be provided with a flange member, such as a nut or a bolt head, having a plurality of recesses distributed about the tension rod and a like plurality of tensioning members which are received in these recesses and extendable from the flange member towards the wall of the hub body to force the flange member away from the wall of the hub body and thereby subject the tension rod to tension.
Naturally, each tensioning member will apply only a fraction of the total tensioning force that has to be applied to the tension rod to ensure a reliable frictional locking of the propeller blade to the hub body. However, the combined tension force applied by the tensioning members can ensure a firm frictional locking without it being necessary to subject the individual tensioning members to more than a small fraction of the force required for a firm locking of the blades of the prior art propellers, where for each fastener device the torque is applied to a single fastener bolt head or to a nut on the fastener bolt.
Because each tensioning member only has to be subjected to a small force, the tensioning of the tension rod can be effected by means of a small tool, such as a power wrench if the tensioning members are screws. In many cases it is therefore possible to carry out the tensioning operation from within the cavity of the hub body; propellers of the kind with which the invention is concerned normally are large enough to permit a mechanic to work with a hand-held power tool inside the hub body when the propeller blade position is adjusted.
The tension rods in such cases can enter into the propeller blade from the side of the blade flange engaging the bearing surface on the hub body. They accordingly need not extend completely through the thickness of the blade flange, which can therefore have a smooth external surface. This also means that the entire flange surface engaging the bearing surface on the hub body is available for the application of the tension rods. If, on the other hand, the tension rods enter into the blade flange from the opposite side, the side exposed to the water, the root of the propeller blade restricts the space available for the application of the tension rods.